Americans think they'll be ready to retire when the time comes, but their savings haven't caught up.

The average American plans to retire at age 62, according to the latest MassMutual State of the American Family survey, which is down from 64 in 2013. On top of planning to retire earlier, 47% of the participants surveyed reported they felt confident that they would be able to retire at their chosen age, up 2% from the 2013 results.

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On the surface, this paints an optimistic picture. But when you dig into the details, the truth becomes less rosy. Only 36% of those surveyed said they had developed a plan for retirement savings, and only 24% of that 36% said they always stuck to their plan.

But without a plan and the commitment to follow it, it's all too easy to get off track. Being unprepared for retirement means you'll have to work longer than you anticipated or otherwise risk running out of money during retirement. Calculating how much you need to save each month in order to achieve your goals can help you avoid these sad outcomes.

Calculate your retirement expenses

It's hard to know exactly how much money you'll need for retirement because there are so many variables. The best you can do is estimate, and reassess your plan periodically to ensure it still fits your needs.

The first step is to estimate how long your retirement will last. Average life expectancy in the U.S. is about 78.7 years, but many factors can influence this, including gender, lifestyle habits, and family health history. Start from this average and adjust it up or down based on how long you expect to live. Then, subtract this amount from your ideal retirement age to figure out how long your retirement will last.

The next and perhaps most complicated step is to calculate your living expenses in retirement. First, add up how much you think your monthly expenses will be, remembering that certain current expenses, like child care, may disappear in retirement. Now, multiply this amount by 12 to get your yearly expenses. Next, multiply this amount by the number of years of your retirement -- remembering to factor in inflation. A safe inflation estimate is about 3% per year. So if your living expenses amount to $50,000 this year, assume $51,500 for next year and so on. Fortunately, you don't have to do all this number crunching on your own as there are plenty of retirement calculators to use.

Now you've got your total amount you need for retirement and finally some good news: you don't have to save all of this money on your own. If you have a 401(k) match from your job, your employer will contribute money for each contribution you make. Plus, you'll most likely be entitled to some money from Social Security, which figures your benefits based on your average monthly earnings during your 35 highest-earning years and your age when you begin claiming benefits.

You can start claiming your Social Security benefits as early as 62, but to reap your full benefits, you must wait until your full retirement age, 66 or 67 for most adults today. You can also delay benefits up until age 70 in exchange for larger checks later on. Create a My Social Security account for an accurate estimate of how much your checks will be. Then, multiply this by the number of years of your retirement and subtract this from your total, along with any employer-matched funds, to figure out how much you have to save on your own.

The final step is to figure out how much you need to save each month in order to hit your target goal. Keep in mind that you will have compound interest on your side, especially if you start saving early, so you won't need to save the exact dollar amount that you'll need in retirement.

It's difficult to predict what your return might be. It could be as high as 8% per year, but if you want to be conservative, 5% is a better number to start with. It's always better to figure too high than too low. Some retirement calculators will tell you how much you need to save per month and you can also calculate the amount yourself.

Don't panic if you're behind

If retirement is still a few decades away, you have time to turn things around. Boost your retirement contributions if you can. You may need to make some adjustments, like cutting back on current spending or even picking up a side job. The sooner you make that correction, the easier everything will be in your golden years. Waiting to save for retirement only increases the percentage of your paycheck that you need to contribute in order to hit your goal. Take advantage of compound interest by saving early and often.

If retirement is quickly approaching, you have fewer options. Adults 50 and older are allowed to make catch-up contributions of an extra $6,000 to a 401(k) and $1,000 to an IRA in 2018, for a total possible contribution of up to $24,500 and $6,500 to a 401(k) and IRA, respectively. If that isn't enough or you can't afford catch-up contributions, you may have to think about pushing your retirement back to give yourself more time to save. It's not ideal, but it beats running out of money in your last years on Earth.

You may very well be able to retire at 62, but it's important to have a plan that gets you there. Otherwise, you may not realize that you're off track until it's too late to do anything about it. By following the outline listed above, you can estimate how much you'll need for retirement and how much you need to save per month to hit your goal.

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